OLED devices are used, for example, to form displays such as flat panel displays (FPDs). The displays are used in many different products such as pagers, cellular phones, and personal organizers. Typically, an OLED comprises one or more organic functional layers sandwiched between two electrodes. Charge carriers are injected through the electrodes and recombine in the functional layers, thereby emitting visible radiation.
One of the electrodes is formed from a transparent conductive material, enabling the radiation to be seen. The transparent conductive material should possess low resistivity, high optical transmittance, and appropriate work function to produce an OLED device with adequate performance.
A transparent electrode material that is useful in OLED applications is indium-tin-oxide (ITO) due to its high transparency in the visible wavelength range. ITO is commonly used in liquid crystal display (LCD) applications. However, resistivity and work function of the ITO used in LCD applications do not meet the requirements for OLED applications. This leads to lower performance in OLED devices, making portable applications which operate on battery power impractical.
Also, the surface morphology (roughness) of ITO layers used in LCD is not suited for OLED applications. Typically, LCD applications require the surface of the ITO to be rough in order to promote adhesion of the polyimide coating. The rough ITO surface produces high electric fields, which can be detrimental for OLEDs. For example, the high electric fields can induce electrical shorts since the electrodes are only separated by a distance of about 100-200 nm (the usual thickness of the organic functional layer stack).
As evidenced from the above discussion, it is desirable to provide a transparent conductive layer that meets the needs for OLED applications.